Limit strains of X70 pipes with a semi-elliptical crack based on initiation and ductile tearing criteria

Crack-tip opening displacement (CTOD) and J-integral have been used for elastic-plastic fracture parameters as a crack driving force (CDF) and crack resistance curve to evaluate tensile strain capacity (TSC) of cracked pipelines based on strain-based design (SBD). The TSC can be determined by using two kinds of failure criteria. One is based on the limit state corresponding to an onset of stable crack growth and the other is tangency approach which determines an onset of unstable crack growth by comparing crack driving force and resistance curve. For this reason, the accurate calculation of crack driving force and crack resistance curve is highly required to determine TSC. In the present study, the TSCs for X70 pipelines with a circumferential semi-elliptical surface crack were estimated based on both crack initiation and ductile tearing criteria using crack driving force diagram (CDFD) method. The CDF curves of cracked pipelines were calculated through the detailed elastic-plastic finite element (FE) analyses. Crack resistance curves were obtained from experimental data of single edged notch tension (SENT) specimens. Both the CDF and crack resistance curves were represented using CTOD and J-integral, respectively. As for loading conditions, axial strain and internal pressure were considered. The TSCs based on CTOD were compared with those based on J-integral to investigate the effect of choice of the fracture parameters on TSC. From the FE results, the TSCs based on ductile tearing allowed higher TSCs than those based on crack initiation. Although there were some differences between the TSCs using CTOD and J-integral, the effect of choice of fracture parameter on TSC with internal pressure was not significant.